This invention relates generally to vapor deposition of materials onto substrates, and more particularly to a method and apparatus for vapor depositing material onto a substrate, which material is contained on a mesh or screen member.
It is conventional prior art practice to vapor deposit many different types of material onto many different substrates. This is done for many different purposes and to obtain many different resultant structures. In one particular application of vapor deposition, solder material is vapor deposited onto a semi-conductor wafer to form solder pads which will form sites to join chips formed from the substrate to a ceramic material. The present invention is particularly adapted for such type of vapor deposition of solder onto semi-conductor substrates although it is not specifically limited to such applications.
In the vapor deposition of solder onto substrates, it has been conventional prior art practice to provide a source of the solder material, which is disposed in a cup constructed of high-temperature alloy or ceramic material which allows the solder material to be melted and evaporated. The cup is conventionally heated by RF induction coils, but other means can be used, and the cup has an opening formed in the top. The melted and subsequently vaporized solder material escapes through the opening in the cup and is deposited onto semi-conductor substrates which are mounted within an enclosure in which the cup is supported. If discrete solder pads are required, then deposition conventionally is performed through a patterned mask, associated with the substrate.
One of the problems associated with this and with other types of vapor deposition techniques is what is known as "spitting" or "spattering". This phenomenon is characterized by fairly large globs of material being expelled from the melt through the opening and thence being deposited onto the substrate or onto the mask. Other difficulties encountered with the vapor deposition technique of using a cup and opening include the difficulty of obtaining uniform distribution of the solder material emanating from the cup. This is especially true in an arrangement wherein a plurality of substrates to be coated are supported in an array in a conventional dome-shaped container. With this particular coating process, uneven distribution can be a significant problem. Other problems encountered include limitation on deposition thickness and limited source volumes.
There have been several prior art proposals to overcome certain of these drawbacks. For example, Hanson, et al, U.S. Pat. No. 3,446,936, discloses, in one of the embodiments therein, a baffling arrangement to intercept spits and prevent spitting from the evaporant source. IBM Technical Disclosure Bulletin, Volume 6, No. 4, September, 1963, Page 1, shows an evaporation source for copper, wherein pin holes are punched through the bottom of the evaporation boat to allow the copper to come out in very small volumes. The problem of uneven deposition is addressed in the Baer U.S. Pat. No. 3,517,644, wherein various resistors to be coated are contained within a cage which is rotated during the vapor deposition.
Certain source configurations are shown in IBM Technical Disclosure Bulletin, Volume No. 9, No. 12, May, 1967, Page 1677; and evaporation mask coating features are disclosed in IBM Technical Disclosure Bulletin, Volume 9, No. 5, October, 1966, Page 543. A technique for controlling the rate of evaporation and measuring the rate is shown in IBM Technical Disclosure Bulletin, Volume No. 6, No. 7, December 1963, Page 77.
However, none of these references suggest the technique of the present invention for significantly reducing or even substantially eliminating the problem of spitting , nor do they suggest the technique for providing more uniform distribution of the vapor deposited material.